One way of burning coal cleanly is in a fluidized bed furnace, where the coal is burned in a bed of inert material such as limestone. These units must be capable of operating at some reduced load range to follow load demand and to permit normal start-up operation. Conventional air distributors for a fluidized bed are designed with a pressure drop equal to about 1/3 of the bed pressure drop to ensure uniform distribution of air over the cross section of the bed. Since the combustor or furnace is normally designed to operate over some range of load, the distributor must be sized to give the desired pressure drop at the minimum load. As a result, when load is increased to maximum, the distributor pressure drop increases over and above the pressure needed for good air distribution. The pressure drop increases with the square of the velocity through the distributor. Thus a distributor designed for four inches water pressure drop at 50% load, will have a pressure drop of sixteen inches water at MCR, or 100% load. This increases the capitol cost of the unit since the fan must be sized for full load conditions. It also greatly increases the power requirements for operating the fan during the lifetime of the unit.